CMOS image sensors that are designed to operate at low illumination levels require very low noise readout circuits. A CMOS imaging sensor typically consists of a two-dimensional array of pixel sensors arranged as a plurality of rows and columns. Each pixel sensor includes a photodiode and a readout circuit that converts the charge accumulated by the photodiode during the image exposure to a voltage that is readout by a readout amplifier that is typically shared by all of the pixel sensors in a column. The pixel sensors in each row are readout in parallel using the column readout amplifiers. To maintain low noise, the time between the transfer of the photocharge to the readout circuits in the pixel sensors and the readout of that charge must be as small as possible, since noise accumulates in the pixel sensors during this phase.
To limit the noise in the column readout amplifiers, a low bandwidth amplifier is typically utilized. The lower the bandwidth of the amplifier, the lower the readout noise from the amplifier. However, lowering the bandwidth of the readout amplifier results in an increase in the time required to readout the pixel currently attached to the readout amplifier, since the reduced bandwidth amplifier takes longer to settle. Hence, there is a tradeoff between readout time and readout noise. Accordingly, reducing the noise in the column readout amplifiers remains a challenge in low light sensors.